36 INTRODUCTION TO SEXUAL PHYSIOLOGY 



whereas all the ova and one-half of the spermatozoa contain 

 twenty-four chromosomes, the other half of the spermatozoa — • 

 those which on conjugating with the ova are supposed to give 

 rise to males — contain twenty-three chromosomes). ^ 



The changes which occur in spermatogenesis may be summar- 

 ised as follows : (1) A sperm mother cell or primary spermatocyte 



Priinarii oociite 

 [I mmaliire oruru) 



© 



Secondare/ onciftc I \ I ^^ \Polarhodii 



. © 



Mature ovum 



Polar bodies 



Fifj. 23. — Diagram illustrating the reduction of the chromosomes in 

 the process of maturation of the ovum. The immature ovum 

 has four chromosomes in the case represented, and the mature 

 ovum two chromosomes, or half the original number. The 

 maturation of the spermatozoon is a similar process, only all the 

 four products of division are equal and become functional 

 spermatozoa. 



divides, in which process the chromosomes do not split, so that 

 each product of division or secondary spermatocyte contains half 

 the original number of chromosomes. (2) A secondary spermato- 

 cyte divides, giving rise to a spermatid, and in this process the 

 chromosomes also divide. Subsequently, the spermatid elongates 



^ As we shall see later in discussing sex-determination, in a large 

 number of animals the reduction process may result in two sorts of 

 spermatozoa being produced ; these after fertilisation give rise respectively 

 to the two sexual individuals in whose body cells the chromosomes 

 differ in number or in kind. 



